Adaptive equalizers are used to compensate for the effects of distortion in the transmission channel of a communications system. The typical equalizer structure is a time-domain network having a tapped delay line in the input signal path, a multiplier connected to each delay line tap for multiplying each tap signal by a tap-weight coefficient, and a summing circuit for combining the products formed by each multiplier into the equalizer output signal. In general, for this structure, the greater the number of taps and multipliers, the more precise the distortion compensation. In addition, since the distortion in the transmission channel is time-varying, the tap-weight coefficients must be adjusted so that the compensation provided tracks the distortion. A number of schemes having varying degrees of accuracy, such as zero forcing and least mean square, are utilized to provide the tap-weight coefficient adjustment.
A number of problems have persisted with the use of adaptive equalizers. First, the number of devices required in the summing network grows exponentially with each additional tap. Second, the cost of the hardware reqired to implement the more accurate tap-weight coefficient adjustment algorithms is prohibitive in certain system applications.